Various moulding processes are known: moulding by casting, by cladding the walls of a mould, by heat forming, blow moulding, or again, injection or compression moulding.
In the case of shaped objects having substantial deformed areas, use is also made of a combination of different processes and, in particular, the technique of injection-compression moulding.
In this case, in a mould of a matching shape is held a sheet of thermoplastic, or textile, material, also known as a "skin", which will form the outer face of the object. Then, into the partially open mould, thermoplastic resin is injected under the said skin to form the inner layer of the object, which layer will give the object volume and holding qualities, as well as a certain strength.
In view of the deformability of the outer skin, the latter is able to mate with the different shapes of the mould and its thickness can be determined to favour this property.
However, there are limits to this process when it is necessary to form an object the outer face of which presents substantial deformed areas which give rise to considerable elongation of the raw materials in order to arrive at the desired shapes, and it is not uncommon to find cracks occurring in the skin through which the inner layer appears.
This is the case, for example, when flat sheets are used to produce panels of door linings with substantial hollow and raised shapes such as locations for decorative panels, integral elbow rests, etc.
Such processes are also unapplicable when using an outer skin having a grained appearance and/or raised patterns. Indeed, if the part in question has substantial deformed areas, whether with sharp edges or otherwise, it is customary to flatten the grain or the patterns in such areas, which impairs the aesthetic appearance of object produced.
The object of the present invention is to provide a process for manufacturing a multilayer object by moulding, the said object having an outer face bearing one or more more or less substantial hollow and/or raised deformed area, which permits use of the injection-compression moulding technique, without this being prejudicial to the aesthetic qualities of the product.
In particular, the process according to the present invention makes it possible to remedy the aforementioned drawbacks and to obtain an outer face of the object of flawless aesthetic appearance, that is to say without causing the appearance of any cracking and/or flattening the patterns on the outer face or harming its grained appearance.
Another object of the present invention is to provide an injection-compression moulded multilayer object one of the advantageous applications of which concerns the field of vehicles and, more precisely, the manufacture of dashboards and/or items of trim for bodywork.
A further object of the present invention is to provide a process for manufacturing a multilayer object the outer face of which can take the form of a skin of small thickness produced, for example, from a noble and more costly material, the volume and holding qualities of the object being imparted by an inner layer, used as a filling material, and less costly.
Further objects and advantages of the present invention will emerge in the course of the description that follows, which is given, however, only by way of illustration and is not intended to limit it.